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cellular: random socket port number #7097

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Merged
merged 2 commits into from Jun 11, 2018
Merged

cellular: random socket port number #7097

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b7bad77
random socket port number
Jun 1, 2018
d3a46eb
more effective port randomisation
Jun 5, 2018
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10 changes: 4 additions & 6 deletions features/cellular/framework/common/CellularUtil.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,7 @@
* limitations under the License.
*/
#include "CellularUtil.h"
#include "randLIB.h"
#include <string.h>
#include <stdlib.h>

Expand Down Expand Up @@ -315,12 +316,9 @@ int char_str_to_hex_str(const char* str, uint16_t len, char *buf, bool omit_lead

uint16_t get_dynamic_ip_port()
{
static uint16_t port;
port++;
if (port < 49152) {
port = 49152;
}
return port;
randLIB_seed_random();
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As you're not tracking which ports you've already chosen or are still using, I think it would probably be better to keep a cycling algorithm to maximise time before reuse in one session.

Using some random online birthday paradox calculator, I find the above has a 50% chance of reuse in 150 calls.

If you want to start digging deeper into changing strategies, read https://tools.ietf.org/html/rfc6056, but for now I suggest sticking with the same cycling strategy and just making sure you start at a random point. Nanostack uses algorithm 5 from that paper, but it knows what the current ports in use are:

#define RANDOM_PORT_NUMBER_START 49152
#define RANDOM_PORT_NUMBER_END 65535
#define RANDOM_PORT_NUMBER_COUNT (RANDOM_PORT_NUMBER_END - RANDOM_PORT_NUMBER_START + 1)
#define RANDOM_PORT_NUMBER_MAX_STEP 500
static uint16_t port_counter;
port_counter = randLIB_get_random_in_range(0, RANDOM_PORT_NUMBER_COUNT - 1);


uint16_t count = RANDOM_PORT_NUMBER_COUNT;

do {
    port_counter += randLIB_get_random_in_range(1, RANDOM_PORT_NUMBER_MAX_STEP);
    while (port_counter >= RANDOM_PORT_NUMBER_COUNT) {
        port_counter -= RANDOM_PORT_NUMBER_COUNT;
    }
    uint16_t port = RANDOM_PORT_NUMBER_START + port_counter;
    if (socket_port_validate(port, protocol) == eOK) {
        return port;
    }
} while (--count > 0);

You could use that, but you'd probably want a smaller MAX_STEP unless you can add a validate.


return (randLIB_get_16bit() | 0xC000);
}

} // namespace mbed_cellular_util